1. Field of the Disclosure
This invention relates generally to wafer-scale assembly (WSA) circuits that include benzocyclobutene (BCB) layers and, more particular, to WSA circuits that include BCB layers, and trench vias and a checkerboard pattern of vias through the BCB layers at a sealing ring edge of the layer to provide a hermetic seal and prevent BCB layer shrinkage.
2. Discussion of the Related Art
It is known in the art to provide wafer-level packages for integrated circuits, such as monolithic millimeter-wave integrated circuits (MMIC), formed on substrate wafers. In one wafer-level packaging design, a cover wafer is formed to the substrate wafer by a sealing ring so as to provide a hermetically sealed cavity in which the integrated circuit is provided. During wafer-scale assembly (WSA), many integrated circuits are formed on the substrate wafer and covered by a single cover wafer, where each integrated circuit is surrounded by a separate sealing ring. The cover wafer and the substrate are then diced between the sealing rings to separate the packages for each separate integrated circuit. The dicing process typically uses a saw that cuts the cover wafer between the packages where a portion of the cover wafer is removed. The substrate wafer is then cut between the packages. For certain wafer-level packaging designs, vias are provided through the substrate wafer that make electrical contact with signal lines and ground lines electrically coupled to the integrated circuit within the cavity.
As is well understood by those skilled in the art, integrated circuit design and WSA packaging allow for many circuit components to be fabricated in a small area. Some of these designs employ stacked metal interconnect layers where several interconnect layers are fabricated on top of each other. It has been proposed in the art to use a BCB layer to separate interconnect layers to increase device performance. The BCB layer is an insulated layer that prevents DC current from traveling from one interconnect layer to another interconnect layer, but allows RF signals to propagate therethrough, which may benefit certain circuit designs where signal line cross talk is desirable. As the number of interconnect layers increases, the complexity of the circuit can increase so that more powerful devices can be provided in a smaller hermetically sealed environment. Particularly, high frequency RF lines can cross over each other if there is a BCB layer between them that prevents a DC short between the lines. By depositing a BCB layer on an interconnect layer on either the substrate wafer or the cover wafer, another interconnect layer can be formed on the BCB layer and be electrically isolated therefrom. Metal vias would be fabricated through the BCB layers to allow interconnections between the various interconnect layers.
The increase in the number of interconnect layers in a WSA circuit enables new types of WSA circuit designs to be implemented, which would otherwise not be possible with standard MMIC or WSA MMIC devices. The insertion of RF shielding between interconnect layers in the vertical direction helps enable these new circuit designs to be realized. Unshielded RF coupling in the vertical direction also enables such designs. The RF shield is a metal layer that prevents both RF and DC signals from traveling from one interconnect layer to another.
The various interconnect layers extend to a sealing ring at an outer edge of the WSA package. However, BCB is not a hermetic material and includes a large amount of tensile stress. In order to provide the hermetic seal at the edge of the sealing ring where the BCB layers are located, an etch would need to be provided that was then filled with a metal via between the interconnect layers on opposite sides of the BCB layer so that hermetic seal of the ring can be established. However, when such a BCB via is formed at the sealing location of the ring, the tensile strength of the BCB material causes it to pull away from the metal, which affects device integrity. More particularly, the etch for the sealing vias in the BCB layers causes the BCB layer to shrink, which may disturb connect vias through the BCB layer at other locations in the assembly.